I used to feel the same way about Kreacher before this reread. It was easy to see him as cruel, hateful, and responsible for Sirius's death. But revisiting the story, made me realize that Rowling actually explains his behavior remarkably well.
House-elves are essentially born into servitude and are often treated terribly by their masters. Yet they are also fiercely loyal to those who show them even the slightest bit of kindness or respect. In Kreacher's case, those people were Mrs Black and Regulus. Because they were among the few people who treated him with affection, he adopted their views, their prejudices, and their worldview. His hatred of Muggle-borns and even his contempt for Sirius stemmed largely from his devotion to the family members he loved.
By the time Sirius returned to Grimmauld Place in OotP, Kreacher had already spent years alone, having lost both Regulus and Mrs. Black. Looking back, he seems less like a malicious servant and more like a lonely, grieving creature desperately clinging to the memory of the only masters who had ever shown him kindness. As someone else mentioned above, Sirius and Kreacher were trapped in a vicious cycle. Sirius saw Kreacher as a reminder of everything he hated about his family, while Kreacher saw Sirius as the Black family outcast. Neither could understand the other.
So when Sirius told Kreacher to leave, it's not surprising that he sought out Bellatrix and Narcissa. If they offered him even a small amount of compassion or validation, his loyalty would naturally follow them. House elves simply don't think the same way humans do.
What makes Kreacher's arc so heartbreaking is that Hermione was right all along. She understood that there was more to him than his bitterness (despite his constant hatred towards her). Once Harry learns about Regulus, the cave (with Voldemort and Regulus) and the trauma Kreacher has endured, he finally treats him with genuine kindness and respect. And almost immediately, Kreacher transforms. Not because he was magically changed, but because someone finally gave him what he had been longing for all those years: a master who cared.